1. Field of the Invention
The present invention relates to a polypropylene resin composition for coating the exterior parts of automobiles which can afford an excellent coating adherence, a high rigidity, high impact strength at low temperatures, prominent heat resistance and an excellent appearance to moldings even without effecting a pretreatment step, i.e., treatment with trichloroethane before coating, in addition without applying a primer as undercoating.
2. Description of the Related Art
Recently automotive parts have been made to have an increasingly thinner wall thickness as an increasingly larger type of automobile having a lighter weight has been manufactured, so that there is a need for coating materials, which enable coatings excellent in heat resistance, impact strength at low temperatures, and in appearance to be produced on moldings without using any primer, and which are also advantageous in cost.
There have been heretofore proposed for such materials numerous compositions comprising a combination of crystalline polypropylenes with rubber and inorganic fillers. However, most exterior automotive parts are likely to be attached in place after painted to have brilliant and high grade perception. Current techniques for the paintings employ the steps of treating moldings with a vapor of 1,1,1-trichloroethane normally for 30 seconds to degrease and etch, then undercoating with a primer, and then applying a polyurethane paint as overcoating.
Japanese Patent KOKAI (Laid-open) No. 63-128045 discloses an improved coating composition comprising a polypropylene polymer composition having an excellent adhesiveness with polyurethanes as overcoating even without using any primer. In this patent, however, a treatment with a vapor of 1,1,1-trichloroethane is required.
The use of 1,1,1-trichloroethane was restricted and decided to be entirely inhibited in 1995, together with the flon gas regulation, by the conference held in London June, 1990. Therefore, coating materials for automotive parts simply having rigidity, strength, heat resistance and impact strength at low temperatures are not sufficient to cope with the regulations unless they allow the treatment with 1,1,1-trichloroethane to be omitted and in addition enhanced coating adherence to be achieved advantageously in cost even without applying any primer. Development of such coating materials has been vigorously sought.
As a result of our intensive research made to overcome the difficulties as described above, it has been found that they can be overcome by using a specific polypropylene resin composition having specific components, based on which the present invention has been completed.